Each of your relationships has a different dynamic, so do not be worried if someone you've been having lots of fun with is suddenly hard to get a hold of. They are just living their life, not trying to avoid you! Your insecurity is completely unwarranted, so don't act on it. The last thing you should do is start demanding more from them. This is just how they are in relationships. If you don't like it, you can't change it: you have to accept them as they are.